LITTLE ROCK, Ark. (AP) - A controversial proposal that would require Arkansas voters to show photo identification before casting a ballot hit a roadblock Monday, as lawmakers clashed over whether the state's constitution requires a two-thirds majority to pass the measure.

Rep. Jim Nickels, D-Sherwood, said the supermajority is required because the bill would change the state's permanent voter registration system established under the Arkansas Constitution. The bill was approved in the Senate on a 23-12 vote, one vote shy of a two-thirds majority.

The constitution's Amendment 51, approved by voters in 1964 to eliminate the poll tax, outlines what documentation the state can demand of residents who want to register to vote. The provision says that the Legislature can only change the voter registration process with a two-thirds majority vote.

Nickels argued that requiring a voter to present photo identification was a "de facto registration requirement."

"It's adding a requirement that's not listed in Amendment 51, and since it in essence then amends (Amendment) 51, then my argument is it takes a supermajority vote to do that," he said.

Nickels said that if the Legislature were to pass the measure by a simple majority, it would expose the state to additional legal challenges. The American Civil Liberties Union of Arkansas has already promised to file a lawsuit to block the legislation if it becomes law.

The bill's sponsor, Sen. Bryan King, R-Green Forest, said he believes the measure requires only a simple majority because it does not address registration requirements. He said the same conclusion was reached by Bureau of Legislative Research lawyers who drafted the bill reached the same conclusion.

House Speaker Davy Carter said he would refer the bill to the House Rules Committee, which he said would likely decide the issue later this week. Carter, who has indicated he supports the bill, said he believes the measure could pass the House with a majority but likely not by a two-thirds vote. Carter agreed that the legal question would turn on whether the voter ID requirement constitutes a registration requirement.

"I don't think there's an easy answer to the question," Carter said.

Nickels acknowledged he opposes the bill regardless, likening the photo ID requirement to a poll tax.

"It's adding something that is going to be expensive for some folks to comply with," he said. "I would not support it in its current form, either by a simple majority or a supermajority."